Forecasting the Constraint on the Hu–Sawicki f ( R ) Modified Gravity in the CSST 3 × 2 pt Photometric Survey

Abstract We forecast the constraint on the Hu–Sawicki f ( R ) model from the photometric survey operated by the Chinese Space Station Survey Telescope (CSST). The simulated 3 × 2 pt data of galaxy clustering, weak lensing, and galaxy–galaxy lensing measurements within 100 deg 2 are used in the analysis. The mock observational maps are constructed from a light cone, redshift sampling, and noise. The angular power spectra are measured with pseudo- C ℓ estimators and compared to theory in the same basis using validated weighting functions and an analytic covariance matrix that includes Gaussian, connected non-Gaussian, and supersample terms. We model the theoretical spectra using two methods. The first one uses MGCAMB to compute the linear modified-gravity clustering power spectra, and the second one adopts the FREmu emulator with a baseline of nonlinear ΛCDM prescription. Parameter inference is performed with Cobaya , and the cosmological and modified-gravity parameters are sampled within the emulator training domain, which are jointly fitted with the systematic parameters. We find that the predictions from the two methods are in good agreement at the overlapping large scales, and the emulator method can correctly provide additional high- ℓ information. The 1 σ upper bounds of log 10 ∣ f R 0 ∣ are found to be